This is a short document describing the preferred coding style for the
linux kernel. Coding style is very personal, and I won’t force my
views on anybody, but this is what goes for anything that I have to be
able to maintain, and I’d prefer it for most other things too. Please
at least consider the points made here.

First off, I’d suggest printing out a copy of the GNU coding standards,
and NOT read it. Burn them, it’s a great symbolic gesture.

Tabs are 8 characters, and thus indentations are also 8 characters.
There are heretic movements that try to make indentations 4 (or even 2!)
characters deep, and that is akin to trying to define the value of PI to
be 3.

Rationale: The whole idea behind indentation is to clearly define where
a block of control starts and ends. Especially when you’ve been looking
at your screen for 20 straight hours, you’ll find it a lot easier to see
how the indentation works if you have large indentations.

Now, some people will claim that having 8-character indentations makes
the code move too far to the right, and makes it hard to read on a
80-character terminal screen. The answer to that is that if you need
more than 3 levels of indentation, you’re screwed anyway, and should fix
your program.

In short, 8-char indents make things easier to read, and have the added
benefit of warning you when you’re nesting your functions too deep.
Heed that warning.

The preferred way to ease multiple indentation levels in a switch statement is
to align the switch and its subordinate case labels in the same column
instead of double-indenting the case labels. E.g.:

Coding style is all about readability and maintainability using commonly
available tools.

The limit on the length of lines is 80 columns and this is a strongly
preferred limit.

Statements longer than 80 columns will be broken into sensible chunks, unless
exceeding 80 columns significantly increases readability and does not hide
information. Descendants are always substantially shorter than the parent and
are placed substantially to the right. The same applies to function headers
with a long argument list. However, never break user-visible strings such as
printk messages, because that breaks the ability to grep for them.

The other issue that always comes up in C styling is the placement of
braces. Unlike the indent size, there are few technical reasons to
choose one placement strategy over the other, but the preferred way, as
shown to us by the prophets Kernighan and Ritchie, is to put the opening
brace last on the line, and put the closing brace first, thusly:

However, there is one special case, namely functions: they have the
opening brace at the beginning of the next line, thus:

intfunction(intx){bodyoffunction}

Heretic people all over the world have claimed that this inconsistency
is ... well ... inconsistent, but all right-thinking people know that
(a) K&R are right and (b) K&R are right. Besides, functions are
special anyway (you can’t nest them in C).

Note that the closing brace is empty on a line of its own, except in
the cases where it is followed by a continuation of the same statement,
ie a while in a do-statement or an else in an if-statement, like
this:

do{bodyofdo-loop}while(condition);

and

if(x==y){..}elseif(x>y){...}else{....}

Rationale: K&R.

Also, note that this brace-placement also minimizes the number of empty
(or almost empty) lines, without any loss of readability. Thus, as the
supply of new-lines on your screen is not a renewable resource (think
25-line terminal screens here), you have more empty lines to put
comments on.

Do not unnecessarily use braces where a single statement will do.

if(condition)action();

and

if (condition)
do_this();
else
do_that();

This does not apply if only one branch of a conditional statement is a single
statement; in the latter case use braces in both branches:

if(condition){do_this();do_that();}else{otherwise();}

Also, use braces when a loop contains more than a single simple statement:

Linux kernel style for use of spaces depends (mostly) on
function-versus-keyword usage. Use a space after (most) keywords. The
notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
somewhat like functions (and are usually used with parentheses in Linux,
although they are not required in the language, as in: sizeofinfo after
structfileinfoinfo; is declared).

So use a space after these keywords:

if, switch, case, for, do, while

but not with sizeof, typeof, alignof, or __attribute__. E.g.,

s=sizeof(structfile);

Do not add spaces around (inside) parenthesized expressions. This example is
bad:

s=sizeof(structfile);

When declaring pointer data or a function that returns a pointer type, the
preferred use of * is adjacent to the data name or function name and not
adjacent to the type name. Examples:

Use one space around (on each side of) most binary and ternary operators,
such as any of these:

= + - < > * / % | & ^ <= >= == != ? :

but no space after unary operators:

& * + - ~ ! sizeof typeof alignof __attribute__ defined

no space before the postfix increment & decrement unary operators:

++ --

no space after the prefix increment & decrement unary operators:

++ --

and no space around the . and -> structure member operators.

Do not leave trailing whitespace at the ends of lines. Some editors with
smart indentation will insert whitespace at the beginning of new lines as
appropriate, so you can start typing the next line of code right away.
However, some such editors do not remove the whitespace if you end up not
putting a line of code there, such as if you leave a blank line. As a result,
you end up with lines containing trailing whitespace.

Git will warn you about patches that introduce trailing whitespace, and can
optionally strip the trailing whitespace for you; however, if applying a series
of patches, this may make later patches in the series fail by changing their
context lines.

C is a Spartan language, and so should your naming be. Unlike Modula-2
and Pascal programmers, C programmers do not use cute names like
ThisVariableIsATemporaryCounter. A C programmer would call that
variable tmp, which is much easier to write, and not the least more
difficult to understand.

HOWEVER, while mixed-case names are frowned upon, descriptive names for
global variables are a must. To call a global function foo is a
shooting offense.

GLOBAL variables (to be used only if you really need them) need to
have descriptive names, as do global functions. If you have a function
that counts the number of active users, you should call that
count_active_users() or similar, you should not call it cntusr().

Encoding the type of a function into the name (so-called Hungarian
notation) is brain damaged - the compiler knows the types anyway and can
check those, and it only confuses the programmer. No wonder MicroSoft
makes buggy programs.

LOCAL variable names should be short, and to the point. If you have
some random integer loop counter, it should probably be called i.
Calling it loop_counter is non-productive, if there is no chance of it
being mis-understood. Similarly, tmp can be just about any type of
variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another
problem, which is called the function-growth-hormone-imbalance syndrome.
See chapter 6 (Functions).

Please don’t use things like vps_t.
It’s a mistake to use typedef for structures and pointers. When you see a

vps_ta;

in the source, what does it mean?
In contrast, if it says

structvirtual_container*a;

you can actually tell what a is.

Lots of people think that typedefs helpreadability. Not so. They are
useful only for:

totally opaque objects (where the typedef is actively used to hide
what the object is).

Example: pte_t etc. opaque objects that you can only access using
the proper accessor functions.

Note

Opaqueness and accessorfunctions are not good in themselves.
The reason we have them for things like pte_t etc. is that there
really is absolutely zero portably accessible information there.

Clear integer types, where the abstraction helps avoid confusion
whether it is int or long.

u8/u16/u32 are perfectly fine typedefs, although they fit into
category (d) better than here.

Note

Again - there needs to be a reason for this. If something is
unsignedlong, then there’s no reason to do

typedef unsigned long myflags_t;

but if there is a clear reason for why it under certain circumstances
might be an unsignedint and under other configurations might be
unsignedlong, then by all means go ahead and use a typedef.

when you use sparse to literally create a new type for
type-checking.

New types which are identical to standard C99 types, in certain
exceptional circumstances.

Although it would only take a short amount of time for the eyes and
brain to become accustomed to the standard types like uint32_t,
some people object to their use anyway.

Therefore, the Linux-specific u8/u16/u32/u64 types and their
signed equivalents which are identical to standard types are
permitted – although they are not mandatory in new code of your
own.

When editing existing code which already uses one or the other set
of types, you should conform to the existing choices in that code.

Types safe for use in userspace.

In certain structures which are visible to userspace, we cannot
require C99 types and cannot use the u32 form above. Thus, we
use __u32 and similar types in all structures which are shared
with userspace.

Maybe there are other cases too, but the rule should basically be to NEVER
EVER use a typedef unless you can clearly match one of those rules.

In general, a pointer, or a struct that has elements that can reasonably
be directly accessed should never be a typedef.

Functions should be short and sweet, and do just one thing. They should
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the
complexity and indentation level of that function. So, if you have a
conceptually simple function that is just one long (but simple)
case-statement, where you have to do lots of small things for a lot of
different cases, it’s OK to have a longer function.

However, if you have a complex function, and you suspect that a
less-than-gifted first-year high-school student might not even
understand what the function is all about, you should adhere to the
maximum limits all the more closely. Use helper functions with
descriptive names (you can ask the compiler to in-line them if you think
it’s performance-critical, and it will probably do a better job of it
than you would have done).

Another measure of the function is the number of local variables. They
shouldn’t exceed 5-10, or you’re doing something wrong. Re-think the
function, and split it into smaller pieces. A human brain can
generally easily keep track of about 7 different things, anything more
and it gets confused. You know you’re brilliant, but maybe you’d like
to understand what you did 2 weeks from now.

In source files, separate functions with one blank line. If the function is
exported, the EXPORT macro for it should follow immediately after the
closing function brace line. E.g.:

In function prototypes, include parameter names with their data types.
Although this is not required by the C language, it is preferred in Linux
because it is a simple way to add valuable information for the reader.

Do not use the extern keyword with function prototypes as this makes
lines longer and isn’t strictly necessary.

Albeit deprecated by some people, the equivalent of the goto statement is
used frequently by compilers in form of the unconditional jump instruction.

The goto statement comes in handy when a function exits from multiple
locations and some common work such as cleanup has to be done. If there is no
cleanup needed then just return directly.

Choose label names which say what the goto does or why the goto exists. An
example of a good name could be out_free_buffer: if the goto frees buffer.
Avoid using GW-BASIC names like err1: and err2:, as you would have to
renumber them if you ever add or remove exit paths, and they make correctness
difficult to verify anyway.

The rationale for using gotos is:

unconditional statements are easier to understand and follow

nesting is reduced

errors by not updating individual exit points when making
modifications are prevented

Comments are good, but there is also a danger of over-commenting. NEVER
try to explain HOW your code works in a comment: it’s much better to
write the code so that the working is obvious, and it’s a waste of
time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
you should probably go back to chapter 6 for a while. You can make
small comments to note or warn about something particularly clever (or
ugly), but try to avoid excess. Instead, put the comments at the head
of the function, telling people what it does, and possibly WHY it does
it.

When commenting the kernel API functions, please use the kernel-doc format.
See the files at Documentation/doc-guide/ and
scripts/kernel-doc for details.

The preferred style for long (multi-line) comments is:

/* * This is the preferred style for multi-line * comments in the Linux kernel source code. * Please use it consistently. * * Description: A column of asterisks on the left side, * with beginning and ending almost-blank lines. */

For files in net/ and drivers/net/ the preferred style for long (multi-line)
comments is a little different.

/* The preferred comment style for files in net/ and drivers/net * looks like this. * * It is nearly the same as the generally preferred comment style, * but there is no initial almost-blank line. */

It’s also important to comment data, whether they are basic types or derived
types. To this end, use just one data declaration per line (no commas for
multiple data declarations). This leaves you room for a small comment on each
item, explaining its use.

That’s OK, we all do. You’ve probably been told by your long-time Unix
user helper that GNUemacs automatically formats the C sources for
you, and you’ve noticed that yes, it does do that, but the defaults it
uses are less than desirable (in fact, they are worse than random
typing - an infinite number of monkeys typing into GNU emacs would never
make a good program).

So, you can either get rid of GNU emacs, or change it to use saner
values. To do the latter, you can stick the following in your .emacs file:

This will make emacs go better with the kernel coding style for C
files below ~/src/linux-trees.

But even if you fail in getting emacs to do sane formatting, not
everything is lost: use indent.

Now, again, GNU indent has the same brain-dead settings that GNU emacs
has, which is why you need to give it a few command line options.
However, that’s not too bad, because even the makers of GNU indent
recognize the authority of K&R (the GNU people aren’t evil, they are
just severely misguided in this matter), so you just give indent the
options -kr-i8 (stands for K&R,8characterindents), or use
scripts/Lindent, which indents in the latest style.

indent has a lot of options, and especially when it comes to comment
re-formatting you may want to take a look at the man page. But
remember: indent is not a fix for bad programming.

Note that you can also use the clang-format tool to help you with
these rules, to quickly re-format parts of your code automatically,
and to review full files in order to spot coding style mistakes,
typos and possible improvements. It is also handy for sorting #includes,
for aligning variables/macros, for reflowing text and other similar tasks.
See the file Documentation/process/clang-format.rst
for more details.

For all of the Kconfig* configuration files throughout the source tree,
the indentation is somewhat different. Lines under a config definition
are indented with one tab, while help text is indented an additional two
spaces. Example:

config AUDIT
bool "Auditing support"
depends on NET
help
Enable auditing infrastructure that can be used with another
kernel subsystem, such as SELinux (which requires this for
logging of avc messages output). Does not do system-call
auditing without CONFIG_AUDITSYSCALL.

Seriously dangerous features (such as write support for certain
filesystems) should advertise this prominently in their prompt string:

Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
reference counts. In the kernel, garbage collection doesn’t exist (and
outside the kernel garbage collection is slow and inefficient), which
means that you absolutely have to reference count all your uses.

Reference counting means that you can avoid locking, and allows multiple
users to have access to the data structure in parallel - and not having
to worry about the structure suddenly going away from under them just
because they slept or did something else for a while.

Note that locking is not a replacement for reference counting.
Locking is used to keep data structures coherent, while reference
counting is a memory management technique. Usually both are needed, and
they are not to be confused with each other.

Many data structures can indeed have two levels of reference counting,
when there are users of different classes. The subclass count counts
the number of subclass users, and decrements the global count just once
when the subclass count goes to zero.

Examples of this kind of multi-level-reference-counting can be found in
memory management (structmm_struct: mm_users and mm_count), and in
filesystem code (structsuper_block: s_count and s_active).

Remember: if another thread can find your data structure, and you don’t
have a reference count on it, you almost certainly have a bug.

Kernel developers like to be seen as literate. Do mind the spelling
of kernel messages to make a good impression. Do not use crippled
words like dont; use donot or don't instead. Make the messages
concise, clear, and unambiguous.

Kernel messages do not have to be terminated with a period.

Printing numbers in parentheses (%d) adds no value and should be avoided.

There are a number of driver model diagnostic macros in <linux/device.h>
which you should use to make sure messages are matched to the right device
and driver, and are tagged with the right level: dev_err(), dev_warn(),
dev_info(), and so forth. For messages that aren’t associated with a
particular device, <linux/printk.h> defines pr_notice(), pr_info(),
pr_warn(), pr_err(), etc.

Coming up with good debugging messages can be quite a challenge; and once
you have them, they can be a huge help for remote troubleshooting. However
debug message printing is handled differently than printing other non-debug
messages. While the other pr_XXX() functions print unconditionally,
pr_debug() does not; it is compiled out by default, unless either DEBUG is
defined or CONFIG_DYNAMIC_DEBUG is set. That is true for dev_dbg() also,
and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
the ones already enabled by DEBUG.

Many subsystems have Kconfig debug options to turn on -DDEBUG in the
corresponding Makefile; in other cases specific files #define DEBUG. And
when a debug message should be unconditionally printed, such as if it is
already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
used.

The kernel provides the following general purpose memory allocators:
kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
vzalloc(). Please refer to the API documentation for further information
about them. Documentation/core-api/memory-allocation.rst

The preferred form for passing a size of a struct is the following:

p=kmalloc(sizeof(*p),...);

The alternative form where struct name is spelled out hurts readability and
introduces an opportunity for a bug when the pointer variable type is changed
but the corresponding sizeof that is passed to a memory allocator is not.

Casting the return value which is a void pointer is redundant. The conversion
from void pointer to any other pointer type is guaranteed by the C programming
language.

The preferred form for allocating an array is the following:

p=kmalloc_array(n,sizeof(...),...);

The preferred form for allocating a zeroed array is the following:

p=kcalloc(n,sizeof(...),...);

Both forms check for overflow on the allocation size n * sizeof(...),
and return NULL if that occurred.

These generic allocation functions all emit a stack dump on failure when used
without __GFP_NOWARN so there is no use in emitting an additional failure
message when NULL is returned.

There appears to be a common misperception that gcc has a magic “make me
faster” speedup option called inline. While the use of inlines can be
appropriate (for example as a means of replacing macros, see Chapter 12), it
very often is not. Abundant use of the inline keyword leads to a much bigger
kernel, which in turn slows the system as a whole down, due to a bigger
icache footprint for the CPU and simply because there is less memory
available for the pagecache. Just think about it; a pagecache miss causes a
disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
that can go into these 5 milliseconds.

A reasonable rule of thumb is to not put inline at functions that have more
than 3 lines of code in them. An exception to this rule are the cases where
a parameter is known to be a compiletime constant, and as a result of this
constantness you know the compiler will be able to optimize most of your
function away at compile time. For a good example of this later case, see
the kmalloc() inline function.

Often people argue that adding inline to functions that are static and used
only once is always a win since there is no space tradeoff. While this is
technically correct, gcc is capable of inlining these automatically without
help, and the maintenance issue of removing the inline when a second user
appears outweighs the potential value of the hint that tells gcc to do
something it would have done anyway.

Functions can return values of many different kinds, and one of the
most common is a value indicating whether the function succeeded or
failed. Such a value can be represented as an error-code integer
(-Exxx = failure, 0 = success) or a succeeded boolean (0 = failure,
non-zero = success).

Mixing up these two sorts of representations is a fertile source of
difficult-to-find bugs. If the C language included a strong distinction
between integers and booleans then the compiler would find these mistakes
for us... but it doesn’t. To help prevent such bugs, always follow this
convention:

If the name of a function is an action or an imperative command,
the function should return an error-code integer. If the name
is a predicate, the function should return a "succeeded" boolean.

For example, addwork is a command, and the add_work() function returns 0
for success or -EBUSY for failure. In the same way, PCIdevicepresent is
a predicate, and the pci_dev_present() function returns 1 if it succeeds in
finding a matching device or 0 if it doesn’t.

All EXPORTed functions must respect this convention, and so should all
public functions. Private (static) functions need not, but it is
recommended that they do.

Functions whose return value is the actual result of a computation, rather
than an indication of whether the computation succeeded, are not subject to
this rule. Generally they indicate failure by returning some out-of-range
result. Typical examples would be functions that return pointers; they use
NULL or the ERR_PTR mechanism to report failure.

The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
only evaluate to 0 or 1, and implicit or explicit conversion to bool
automatically converts the value to true or false. When using bool types the
!! construction is not needed, which eliminates a class of bugs.

When working with bool values the true and false definitions should be used
instead of 1 and 0.

bool function return types and stack variables are always fine to use whenever
appropriate. Use of bool is encouraged to improve readability and is often a
better option than ‘int’ for storing boolean values.

Do not use bool if cache line layout or size of the value matters, as its size
and alignment varies based on the compiled architecture. Structures that are
optimized for alignment and size should not use bool.

If a structure has many true/false values, consider consolidating them into a
bitfield with 1 bit members, or using an appropriate fixed width type, such as
u8.

Similarly for function arguments, many true/false values can be consolidated
into a single bitwise ‘flags’ argument and ‘flags’ can often be a more
readable alternative if the call-sites have naked true/false constants.

Otherwise limited use of bool in structures and arguments can improve
readability.

The header file include/linux/kernel.h contains a number of macros that
you should use, rather than explicitly coding some variant of them yourself.
For example, if you need to calculate the length of an array, take advantage
of the macro

#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

Similarly, if you need to calculate the size of some structure member, use

#define FIELD_SIZEOF(t, f) (sizeof(((t*)0)->f))

There are also min() and max() macros that do strict type checking if you
need them. Feel free to peruse that header file to see what else is already
defined that you shouldn’t reproduce in your code.

Do not include any of these in source files. People have their own personal
editor configurations, and your source files should not override them. This
includes markers for indentation and mode configuration. People may use their
own custom mode, or may have some other magic method for making indentation
work correctly.

In architecture-specific code, you may need to use inline assembly to interface
with CPU or platform functionality. Don’t hesitate to do so when necessary.
However, don’t use inline assembly gratuitously when C can do the job. You can
and should poke hardware from C when possible.

Consider writing simple helper functions that wrap common bits of inline
assembly, rather than repeatedly writing them with slight variations. Remember
that inline assembly can use C parameters.

You may need to mark your asm statement as volatile, to prevent GCC from
removing it if GCC doesn’t notice any side effects. You don’t always need to
do so, though, and doing so unnecessarily can limit optimization.

When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate quoted
string, and end each string except the last with \n\t to properly indent
the next instruction in the assembly output:

Wherever possible, don’t use preprocessor conditionals (#if, #ifdef) in .c
files; doing so makes code harder to read and logic harder to follow. Instead,
use such conditionals in a header file defining functions for use in those .c
files, providing no-op stub versions in the #else case, and then call those
functions unconditionally from .c files. The compiler will avoid generating
any code for the stub calls, producing identical results, but the logic will
remain easy to follow.

Prefer to compile out entire functions, rather than portions of functions or
portions of expressions. Rather than putting an ifdef in an expression, factor
out part or all of the expression into a separate helper function and apply the
conditional to that function.

If you have a function or variable which may potentially go unused in a
particular configuration, and the compiler would warn about its definition
going unused, mark the definition as __maybe_unused rather than wrapping it in
a preprocessor conditional. (However, if a function or variable always goes
unused, delete it.)

Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
symbol into a C boolean expression, and use it in a normal C conditional:

if(IS_ENABLED(CONFIG_SOMETHING)){...}

The compiler will constant-fold the conditional away, and include or exclude
the block of code just as with an #ifdef, so this will not add any runtime
overhead. However, this approach still allows the C compiler to see the code
inside the block, and check it for correctness (syntax, types, symbol
references, etc). Thus, you still have to use an #ifdef if the code inside the
block references symbols that will not exist if the condition is not met.

At the end of any non-trivial #if or #ifdef block (more than a few lines),
place a comment after the #endif on the same line, noting the conditional
expression used. For instance: